Facing elite players can be intimidating.

Practicing in the shadow of a Grand Slam champion can be a major diversion—even when you're the best player in the world.

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Such was the case for Serena Williams on Monday at the US Open.

The world No. 1 was practicing on an adjacent court to second-seeded Roger Federer and admits "it's super distracting" trying to keep your eye on the ball while sneaking "side eye" peeks over at the Grand Slam king.

"It is [distracting]  because I'm always looking over, What is he doing? Giving him the side eye," Serena told the media of Federer. "Okay, maybe I should do that. I should do that. Oh, he's taking a break now. Okay, cool. No, it's super distracting. I totally look at him. I mean, he's Roger. Like watching his returns, okay, so he is moving forward on that. Interesting."

The owner of two Serena Slams had a different reaction facing Federer's Davis Cup teammate, Stan Wawrinka.

When the reigning Roland Garros champions squared off in a pre-tournament ping-pong match, Serena concedes she completely spaced out on rule No. 1 of facing Stan.

In ping pong or tennis, don't challenge Sir Stanimal's one-handed backhand or you could get "destroyed."

"He destroyed me. I kept hitting to his backhand. I'm like, Why am I hitting to this guy's backhand?" Serena said. "I wouldn't hit to his backhand in tennis, so that wasn't really smart of me. I'm like, God, the guy never misses, of course. So I tried to move the ball around. But gosh, I got destroyed. I need to practice my ping pong."